We have described "fracture-label" techniques that permit direct cytochemical labeling of freeze-fractured plasma and intracellular membranes as well as cross fractured cytoplasm. We report here the use of "fracture-label" to investigate the distribution and partition of wheat germ agglutinin (WGA) receptor sites over the protoplasmic (P) and exoplasmic (E) plasma membrane faces of freeze-fractured human thymus derive (T) lymphocytes. All exoplasmic faces are strongly labeled by WGA. In contrast, the protoplasmic faces exhibit remarkable variation, ranging from virtual absence of label in some faces to very high densities in other faces. We interpret the presence of WGA receptor sites over the protoplasmic faces to reflect the presence of transmembrane, WGA binding sialoglycoproteins that, during freeze-fracture, partition with the inner half of the plasma membrane. Our results, therefore, indicate heterogeneous expression of integral membrane proteins within populations of human T cells. Fracture-label techniques thus represent an additional tool in the definition of lymphocyte subpopulations.